1. Field of the Invention
The invention relates generally to apparatus and methods for acoustically determining wellbore conditions in cased and uncased bore holes. In more particular aspects, the invention relates to acoustic scanning tools for conducting ultrasonic inspections and evaluations.
2. Description of the Related Art
In conjunction with drilling operations, wireline logging tools are used to inspect the cased and uncased portions of a borehole. These logging tools typically incorporate ultrasonic transducers which, through known pulsed echo techniques, are able to derive information concerning the condition of the wellbore. In a cased wellbore, the logging tools are used to determine the thickness of casing, the presence of damaged casing, the internal diameter of the casing and the condition of the cement bond along the casing's outer diameter. In uncased portions of a wellbore, the wireline logging tools are used to determine features in the bore hole wall such as dips, holes and cracks.
There are basically two types of logging tools in use today. A scanning type tool rotates a transducer so that the transducer scans the borehole circumferentially. There are also fixed, or non-rotatable tools which use numerous fixed transducers, each of which is trained upon an angular segment of the borehole's circumference. An example of a fixed type of tool is the Pulse Echo Tool (PET) available from Halliburton Company. PET is primarily designed as a tool for use in cased boreholes. It contains eight ultrasonic transducers which are disposed along the length of a long cylindrical housing. The transducers are also spaced from each other angularly so that each of the transducers surveys a different portion of the wellbore casing. Currently, the transducers are angularly spaced at 45.degree. increments about the circumference of the housing. The longitudinal spacing of the transducers is necessary since the diameter of the housing for the PET tool is not large enough to accommodate placement of more than one transducer.
The transducers of the PET measure the bond of the cement to the casing. They can also determine the actual thickness of the casing, detect channels and measure inner casing corrosion. The transducers of the PET can be adjusted radially inwardly or outwardly with respect to the tool's housing in order to achieve an optimum standoff. However, this operation must be accomplished by using a special tool to unscrew and remove a threaded transducer retainer. Then a locating spring, or split ring, must be placed into a desired slot on the transducer, the transducer replaced, and the transducer retainer replaced and retightened. In a typical logging operation using the PET tool, the PET tool is moved slowly upward (or possibly downward) through the wellbore. The system electronics "fire" the transducers (or obtain information readings) in a periodic manner so that relatively continuous readings are available for each discrete portion of the casing being logged. Fixed tools, such as the PET, are useful in areas of a borehole which are cased, but they are generally not suitable for use in uncased portions of a borehole. This is because the eight individual transducers provide eight individual sets of readings. There is a discontinuity between each of these sets. In an uncased wellbore portion, the coverage provided by transducer readings must be relatively continuous and cover virtually all areas of the inside of the borehole in order to be useful.
An example of a scanning type tool is the Halliburton CAST-V.TM.. The CAST-V.TM. incorporates a wireline borne scanner sub which contains a motor. The motor rotates a transducer head at the lower end of the scanner sub. The transducer head contains a single transducer which is used to take periodic readings concerning the condition of the cased borehole as the head is rotated. A fixed weighted plug is located directly opposite the transducer on the transducer head and serves to counterbalance the weight of the transducer. Both the transducer and the plug are disposed within the head in a fixed manner and cannot be adjusted radially outwardly or inwardly with respect to the transducer head. As a result, the CAST-V.TM. system is sold with a number of different diameter heads, and the operator selects the head which best places the transducer at an optimum standoff from the casing wall. For example, a CAST-V.TM. system might be supplied with a 3-5/8 inch diameter head, 4-3/8 inch diameter head, 5-5/8 diameter head and 7 inch diameter head.
A proper standoff distance is important since logging systems which incorporate transducers are vulnerable to mud attenuation effects. Too small a spacing between the face of the transducer and the borehole wall causes secondary transmissions to interfere with the reflections of interest. On the other hand, if the spacing between the face of the transducer and the borehole wall is too great, the transducer's signals will attenuate greatly, particularly where oil-based borehole fluids are being used. Therefore, it is important that the transducer face be located an optimum distance from a borehole wall. For this reason, a number of different sized heads are provided with the CAST-V.TM. system. The borehole size may sometimes change as investigation is conducted, even within a single well. When this occurs, the wireline tool must be removed from the well, the head then replaced, the scanner evacuated and refilled with pressure compensation fluid, and the wireline tool rerun into the wellbore.
In a tool having a rotatable head, a balanced head weight is also important. If one portion of the head is heavier than another, the entire scanning tool tends to move and deviate from its longitudinal axis as the head is rotated. The tool, therefore, oscillates slightly during the scanning operation. Oscillation of the scanning tool in this manner causes the time for signal transit (i.e., the time for signals emitted from the transducers to bounce back to the transducers) to vary, potentially affecting the validity of the readings obtained.
The present invention addresses the problems of prior art.